NOx emissions from diesel engines are an environmental problem. Several countries, including the United States, have long had regulations pending that will limit NOx emissions from trucks and other diesel-powered vehicles. Manufacturers and researchers have already put considerable effort toward meeting those regulations.
In gasoline powered vehicles that use stoichiometric fuel-air mixtures, three-way catalysts have been shown to control NOx emissions. In diesel-powered vehicles, which use compression ignition, the exhaust is generally too oxygen-rich for three-way catalysts to be effective.
Several solutions have been proposed for controlling NOx emissions from diesel-powered vehicles. One set of approaches focuses on the engine. Techniques such as exhaust gas recirculation and partially homogenizing fuel-air mixtures are helpful, but these techniques alone will not eliminate NOx emissions. Another set of approaches remove NOx from the vehicle exhaust. These include the use of lean-burn NOx catalysts, selective catalytic reduction (SCR), and lean NOx traps (LNTs).
Lean-burn NOx catalysts promote the reduction of NOx under oxygen-rich conditions. Reduction of NOx in an oxidizing atmosphere is difficult. It has proved challenging to find a lean-burn NOx catalyst that has the required activity, durability, and operating temperature range. Lean-burn NOx catalysts also tend to be hydrothermally unstable. A noticeable loss of activity occurs after relatively little use. Lean-burn NOx catalysts typically employ a zeolite wash coat, which is thought to provide a reducing microenvironment. The introduction of a reductant, such as diesel fuel, into the exhaust is generally required and introduces a fuel economy penalty of 3% or more. Currently, peak NOx conversion efficiencies for lean-burn catalysts are unacceptably low.
SCR refers to selective catalytic reduction of NOx by ammonia. The reaction takes place even in an oxidizing environment. The NOx can be temporarily stored in an adsorbant or ammonia can be fed continuously into the exhaust. SCR can achieve high levels of NOx reduction, but there is a disadvantage in the lack of infrastructure for distributing ammonia or a suitable precursor. Another concern relates to the possible release of ammonia into the environment.
LNTs are NOx adsorbers combined with catalysts for NOx reduction. The adsorbant is typically an alkaline earth oxide adsorbant, such as BaCO3 and the catalyst is typically a precious metal, such as Pt or Ru. In lean exhaust, the catalyst speeds oxidizing reactions that lead to NOx adsorption. Accumulated NOx is removed by creating a rich environment within the LNT through the introduction of a reductant. In a rich environment, the catalyst activates reactions by which adsorbed NOx is reduced and desorbed, preferably as N2. The process of removing accumulated NOx from the LNT is commonly referred to as regeneration, although it may also be referred to as denitration in order to distinguish desulfation, described below.
In addition to accumulating NOx, LNTs accumulate SOx. SOx is the combustion product of sulfur present in ordinarily diesel fuel. Even with reduced sulfur fuels, the amount of SOx produced by diesel combustion is significant. SOx adsorbs more strongly than NOx and necessitates a more stringent, though less frequent, regeneration. Desulfation requires elevated temperatures as well as a reducing atmosphere.
The conditions for denitration may be created in several ways. One approach uses the engine to create a rich fuel-air mixture. This may be accomplished, for example, by injecting extra diesel fuel into one or more engine cylinders after combustion and substantial decompression. Reductant may also be injected into the exhaust downstream of the engine. In either case, a portion of the reductant must be expended to consume oxygen in the exhaust. The reductant can consume oxygen either by reactions in the LNT or by reactions in an upstream unit. For example, U.S. Patent Pub. No. 2004/0050037 describes an exhaust system with a fuel reformer placed inline with the exhaust and upstream of an LNT. The fuel reformer not only consumes excess oxygen, but converts diesel fuel into more reactive reformate. There is a significant fuel penalty, regardless of which approach is used.
Many publications propose reducing the fuel penalty by providing two or more LNTs in a parallel arrangement. During regeneration of an LNT, all or part of the exhaust flow can be diverted to the other LNTs. The implementation of this method requires the use of at least one exhaust valve that for a heavy duty truck must generally fit an exhaust pipe with an inner diameter of at least about 10 cm. U.S. Pat. No. 6,820,417 describes a four-way valve for this purpose. U.S. Patent Pub. No. 2004/0139730 describes a valve that divides reductant and exhaust between two LNTs. In a first position the valve directs reductant to one LNT and exhaust to the other and in a second position switches the flows. The durability and reliability of these valves is not known, although experience with smaller EGR valves suggest durability and reliability will present challenges for these valves.
In certain applications that employ LNTs, as in lean-burn gasoline engines, stoichiometric air-fuel ratios occur during normal operation. It is known to preferentially carry out denitration when such favorable conditions occur during normal vehicle operation. For example, U.S. Patent Pub. No. 2003/0115858, teaches preferentially regenerating an LNT when engine power demand is high, and U.S. Patent Pub. No. 2003/0089103 teaches avoiding regeneration when an engine is at idle.
It is also known that regeneration, especially desulfation, can be carried out more efficiently if initiated while an LNT is relatively hot. U.S. Pat. No. 6,128,899 teaches regenerating a LNT just before fuel cut-off events to avoid having to regenerate the LNT after it becomes cold. U.S. Pat. No. 6,637,198 teaches carrying out partial desulfation when an LNT is at a critical temperature as a result of a normal driving cycle.
U.S. Pat. No. 6,742,328, suggests reducing the fuel penalty for regenerating a LNT in an exhaust treatment system of a compression ignition diesel engine by performing partial regenerations during deceleration to take advantage of low flow conditions.
U.S. Pat. No. 6,732,507 and U.S. Patent Pub. No. 2004/0076565 suggest improving the efficiency of an LNT by combining it with a downstream SCR catalyst in series. The SCR catalyst captures ammonia produced by the LNT toward the end of a complete LNT regeneration cycle. The SCR catalyst subsequently uses the captured ammonia to convert NOx bypassing the LNT and thereby increases the overall extent of NOx conversion.
In spite of advances, there continues to be a long felt need for an affordable and reliable exhaust treatment system that is durable, has a manageable operating cost (including fuel penalty), and can practically be used to reduce NOx emissions across the spectrum of diesel engines to a satisfactory extent in the sense of meeting U.S. Environmental Protection Agency (EPA) regulations effective in 2010 and other such regulations.